1. Field of the Invention
The present invention relates to a DC-DC converter, and more particularly to a technology for protecting a circuit by intermittently turning on and off a switching element when overloaded.
2. Description of the Related Art
FIG. 1 is a diagram showing a circuit configuration of a conventional flyback DC-DC converter. The DC-DC converter is operated by inputting a DC voltage obtained by rectifying and smoothing output from an AC power supply 1 by use of a full-wave rectifier 2 and a capacitor 3.
The DC-DC converter includes: an activation circuit 4; a transformer 5 having a primary winding 5-1, a secondary winding 5-2 and a tertiary winding 5-3; a switching element 6 formed of, for example, a MOSFET; a resistor 7 for detecting an input current; a control circuit 8 for controlling a turning on and off of the switching element 6; a first rectifying and smoothing circuit including a diode 11 and a capacitor 12; a second rectifying and smoothing circuit including a diode 9 and a capacitor 10; an output voltage detection circuit 24; and a photocoupler 13 including a light emitting diode 13-1 and a phototransistor 13-2. The output voltage detection circuit 24 includes resistors 17 and 16 each for detecting an output voltage, an error amplifier 14, and a reference power supply 15.
The primary winding 5-1 of the transformer 5, the switching element 6 and the resistor 7 are connected in series across the capacitor 3. The first rectifying and smoothing circuit is connected to the secondary winding 5-2 of the transformer 5, and the second rectifying and smoothing circuit is connected to the tertiary winding 5-3 of the transformer 5. The activation circuit 4 is connected between a positive terminal of the capacitor 3 and a power input terminal of the control circuit 8 (an output terminal of the second rectifying and smoothing circuit).
The resistors 17 and 16 of the output voltage detection circuit 24 are connected in series between output terminals of the first rectifying and smoothing circuit. An inverting input terminal of the error amplifier 14 is connected to a connection point between the resistors 17 and 16, and a non-inverting input terminal thereof is connected to the reference power supply 15. The light emitting diode 13-1 of the photocoupler 13 is connected to an output terminal of the error amplifier 14. A DC power supply is supplied to the control circuit 8 from the activation circuit 4 or the second rectifying and smoothing circuit. A signal from the phototransistor 13-2 of the photocoupler 13 and a signal from a connection point between the resistor 7 and the switching element 6 are inputted to the control circuit 8. Output from the control circuit 8 is sent to the switching element 6.
Next, operations of the conventional DC-DC converter thus configured will be described. First, operations in a normal state will be described. When the switching element 6 is turned on, energy is stored in the primary winding 5-1 of the transformer 5. When the switching element 6 is turned off, flyback energy of the transformer 5 is transmitted to the secondary winding 5-2. A voltage generated in the secondary winding 5-2 is rectified and smoothed by the first rectifying and smoothing circuit including the diode 11 and the capacitor 12, which are connected to the secondary winding 5-2. Accordingly, DC power is supplied to a load 18.
A DC voltage outputted from the first rectifying and smoothing circuit (a voltage between both ends of the capacitor 12) is divided by the resistors 16 and 17, and is supplied to the inverting input terminal of the error amplifier 14. The error amplifier 14 compares the voltage inputted to the inverting input terminal with a voltage of the reference power supply 15, and outputs an error voltage. Thus, a current flows into the light emitting diode 13-1 of the photocoupler and the light emitting diode 13-1 emits light. The emitted light is transmitted to the phototransistor 13-2, and the phototransistor 13-2 converts the emitted light into an error signal, and sends the signal to the control circuit 8. The control circuit 8 controls the turning on and off of the switching element 6 by generating a PWM signal whose pulse width is adjusted according to the error signal from the phototransistor 13-2 and by sending the signal to a gate of the switching element 6.
In this respect, in a case where the voltage inputted to the inverting input terminal of the error amplifier 14 is larger than a reference voltage outputted from the reference power supply 15, the output from the error amplifier 14 is lowered and the current flowing into the photocoupler 13 is increased. Accordingly, the control circuit 8 generates a PWM signal such as to shorten a time period during which the switching element 6 is being turned on, and sends the signal to the switching element 6. Consequently, an output voltage is lowered, and this is regulated to be a predetermined stabilized voltage.
Next, how the control circuit 8 operates when activated will be described. First, the capacitor 10 is charged through the activation circuit 4 from the capacitor 3. Accordingly, when the voltage of the capacitor 10 is increased to reach an activating voltage Vs that is a voltage at which the control circuit 8 can be operated, the control circuit 8 sends the PWM signal to the switching element 6. Accordingly, the turning on and off of the switching element 6 is started, and a DC voltage is supplied to the load 18 through the first rectifying and smoothing circuit from the secondary winding 5-2. At the same time, a voltage is also generated in the tertiary winding 5-3 of the transformer 5. The generated voltage is rectified and smoothed by the second rectifying and smoothing circuit including the diode 9 and the capacitor 10, and thus the voltage is converted into a DC voltage. The DC voltage is supplied to the control circuit 8 as a power supply for the control circuit 8.
Next, how operations are performed when overloaded will be described. In the above-described state, when lowered impedance of the load 18 increases an output current, a current flowing into the switching element 6 is also increased. The current flowing into the switching element 6 is converted into a voltage by the resistor 7, and the converted voltage is sent to the control circuit 8. The control circuit 8 turns off the switching element 6 when the voltage generated by the resistor 7 gets larger than a predetermined value. Accordingly, since the energy stored in the primary winding 5-1 of the transformer 5 is limited, the flyback energy transmitted to the secondary winding 5-2 is limited. Accordingly, an increase in output power is suppressed, an output voltage is lowered, and overloading is prevented.
However, in a configuration for suppressing an overcurrent flowing into the load 18 by detecting the current flowing into the switching element 6, when the output voltage is lowered, the output current is increased. In particular, when the load 18 is short-circuited or the like, the output current is increased, which may damage the diode 11 and the like. Accordingly, a current limiting circuit may be used, which directly detects the output current and limits the output current so as to set the detected current to be less than a predetermined value.
FIG. 2 is a diagram showing a circuit at a secondary side of another conventional DC-DC converter. In the DC-DC converter shown in FIG. 2, an overcurrent detection circuit 25 is added to the secondary side of the circuit of the DC-DC converter shown in FIG. 1. The overcurrent detection circuit 25 includes a resistor 23, which is interposed between the load 18 and the first rectifying and smoothing circuit, an error amplifier 21, and a reference power supply 22. An inverting input terminal of the error amplifier 21 is connected to a connection point between the resistor 23 and the load 18, and a non-inverting input terminal thereof is connected to the reference power supply 22. Output from the error amplifier 21 is connected to the light emitting diode 13-1 through a diode 20. In order to avoid collision between the output from the error amplifier 21 and the output from the error amplifier 14, a diode 19 is interposed between the error amplifier 14 and the light emitting diode 13-1.
In the conventional DC-DC converter thus configured, a voltage generated by an output current flowing into the resistor 23 is applied to the inverting input terminal of the error amplifier 21. The error amplifier 21 compares the voltage inputted to the inverting input terminal with a reference voltage from the reference power supply 22, which is inputted to the non-inverting input terminal, and increases a current flowing into the light emitting diode 13-1 when the voltage inputted to the inverting input terminal exceeds the reference voltage from the reference power supply 22. The control circuit 8 generates a PWM signal such as to shorten a time period during which the switching element 6 is turned on, according to a signal from the phototransistor 13-2, and sends the signal to the switching element 6. Thus, the output current is limited to a constant current, and the output voltage is lowered. At the same time, a voltage generated in the tertiary winding 5-3 is also lowered, and a power supply voltage supplied to the control circuit 8 is lowered.
When the power supply voltage supplied to the control circuit 8 is lowered to a stop voltage Vu that stops the operations of the control circuit 8, the control circuit 8 stops its operations, and the turning on and off of the switching element 6 stops. When the above state is set, the capacitor 10 is charged by the activation circuit 4 and its voltage increases. A current outputted from the activation circuit 4 is set smaller than a current consumed when the control circuit 8 is operated. On account of this, when the control circuit 8 is operated, the activation circuit 4 cannot charge the capacitor 10 to increase the voltage. However, when the control circuit 8 is stopped, the current consumption is very small. Thus, the activation circuit 4 can charge the capacitor 10 and increase its voltage.
When the voltage of the capacitor 10 is increased to a activating voltage Vs of the control circuit 8, the control circuit 8 restarts its operations, and the switching element 6 is started to be turned on and off. In this event, when the impedance of the load 18 is still low, a large output current flows and the overcurrent detection circuit 25 is operated again to lower an output voltage. When the voltage of the capacitor 10 is lowered to the stop voltage Vu of the control circuit 8, the operations of the control circuit 8 are stopped again.
By repeating the operations described above, the switching element 6 is intermittently turned on and off. According to the configuration described above, the switching element 6 is intermittently turned on and off when the overload state is set. Thus, even if the load 18 is short-circuited or the like, an excessive output current does not flow and components are not damaged. When the impedance of the load 18 is increased and the overload state is ended, the overcurrent detection circuit 25 does not operate even if the turning on and off of the switching element 6 is started. Thus, neither the output voltage nor the power supply voltage of the control circuit 8 is lowered. Consequently, a stable voltage can be supplied to the load 18.
However, in the conventional DC-DC converter shown in FIG. 2 described above, when the current consumed by the control circuit 8 is small, the decreasing speed of the voltage of the capacitor 10 becomes slow. On account of this, the switching operation cannot be immediately stopped, which may cause a flow of an excessive output current. In order to solve this problem, a DC-DC converter disclosed in Japanese Patent Laid-Open Official Gazette No. 2001-145344 surely stops the switching operation, when an overcurrent protection function is operated and the power supply voltage of the control circuit 8 is lowered, by detecting that the power supply voltage reaches a predetermined voltage and forcibly lowering the power supply voltage of the control circuit 8.
FIG. 3 is a circuit configuration diagram of the DC-DC converter disclosed in Japanese Patent Laid-Open Official Gazette No. 2001-145344. The DC-DC converter is formed by adding a pseudo-impedance circuit 26 to the DC-DC converter shown in FIG. 1. The pseudo-impedance circuit 26 is connected between output terminals of the second rectifying and smoothing circuit. The pseudo-impedance circuit 26 includes a backflow prevention diode 51, a Zener diode 52, two resistors 53 and 54, two NPN transistors 55 and 56, and a dummy resistor 57.
In the above configuration, when the impedance of the load 18 is significantly lowered, a DC output voltage supplied to the load 18 from the first rectifying and smoothing circuit is lowered, and a voltage generated in the tertiary winding 5-3 of the transformer 5 is also lowered. Moreover, a power supply voltage supplied to the control circuit 8 through the second rectifying and smoothing circuit from the tertiary winding 5-3 of the transformer 5 is also lowered. When the power supply voltage of the control circuit 8, which is outputted through the second rectifying and smoothing circuit from the tertiary winding 5-3 of the transformer 5, in other words, a rectified voltage obtained through the backflow prevention diode 51 from the tertiary winding 5-3 of the transformer 5, is lowered below a voltage in a normal operation, the Zener diode 52 is set in a non-conducting state. On account of this, the transistor 55 is turned off, and the transistor 56 is turned on. Accordingly, since charge stored in the capacitor 10 is immediately released through the resistor 57 and the transistor 56, the power supply voltage of the control circuit 8 rapidly drops to the stop voltage Vu of the control circuit 8. Accordingly, the operations of the control circuit 8 are stopped, and the DC output voltage comes substantially to 0V. Consequently, the DC-DC converter is stopped.
According to the DC-DC converter described above, when the impedance of the load 18 is significantly lowered, the power supply voltage of the control circuit 8, which is outputted from the second rectifying and smoothing circuit, rapidly drops to the stop voltage Vu of the control circuit 8 by the pseudo-impedance circuit 26. Therefore, even if power consumption of the control circuit 8 is small, the switching operation can be surely stopped. Thus, it is possible to prevent the flow of the excessive output current.
According to the conventional DC-DC converter described above, an overcurrent protection circuit is operated when overloaded, and lowering of the output voltage is detected. Accordingly, the switching element is intermittently turned on and off. Thus, it is possible to prevent the flow of overcurrent when the load is short-circuited and also to reduce the power consumption. In order to reduce the power consumption, it is preferable that an intermittent period of intermittently turning on and off the switching element is set longer.
However, the intermittent period is determined by the current outputted from the activation circuit 4 and the capacity of the capacitor 10. Accordingly, in a case where the output current of the load at the time when the load is short-circuited is reduced by extending the intermittent period, an activating time period for activating the control circuit 8 is also extended. This causes a problem that it may be impossible to satisfy a regulation on the activating time period, which is provided for specifications of the DC-DC converter.